Fix deletion of VCA with slaved controls.

The crashed previously because:
 A VCA is-a Automatable is-a Evoral::ControlSet

 After VCA's d'tor completes ~Automatable runs and emits signal to
 DropReferences of all master-controls.

 This in turn calls SlavableAutomationControl::master_going_away()
 for slaved parameters for the given master-control

 In ::master_going_away() the weak-pointer reference to the master's
 AutomationControl (owned by the destroyed VCA) is still valid.

 Execution is in the d'tor of Automatable which is-a ControlSet and
 the ContolSet keeps a reference to the Control and hence also to the
 AutomationControl which is-a Evoral::Control.

 So master_going_away() locks a boost::shared_ptr<ARDOUR::AutomationControl>
 which is actually the MuteControl owned by the VCA.
 It calls SlavableAutomationControl::remove_master() which
 in turn calls MuteControl::pre_remove_master() which uses the
 MuteMaster API to retrieve the value. The MuteMaster however is the
 VCA that has just been destroyed.


The solution is twofold:
 1) emit "drop_references" from the VCA d'tor itself,
    before the VCA is destroyed.

 2) disconnect a slaved control from the master's drop_references signal
    when un-assigning a master-control.

1 files changed, 1 insertions, 1 deletions

diff --git a/libs/ardour/ardour/automation_control.h b/libs/ardour/ardour/automation_control.h
index cb3b6085f4..e338cb13c1 100644
--- a/libs/ardour/ardour/automation_control.h
+++ b/libs/ardour/ardour/automation_control.h
@@ -62,7 +62,7 @@ class LIBARDOUR_API AutomationControl
 	                  PBD::Controllable::Flag                   flags=PBD::Controllable::Flag (0)
 		);
 
-	~AutomationControl ();
+	virtual ~AutomationControl ();
 
 	boost::shared_ptr<AutomationList> alist() const {
 		return boost::dynamic_pointer_cast<AutomationList>(_list);